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Stable isotope technique is important for understanding the structure and function of soil food web, which is considered as a belowground black box. We reviewed typical application cases of stable isotope techniques in the research of soil food webs, including to determine food sources and feeding preferences of soil fauna by using isotopes, and to analyze the trophic structure of soil food webs through isotope fractionation effects during the process of feeding and nutrient sequestration by soil fauna. Additionally, stable isotope techniques could reveal the role of soil biota at different trophic levels within soil food web in ecosystem matter and energy flow, which favored to carry out accurate and efficient research on the contribution of soil food webs to soil carbon and nitrogen cycling process and the corresponding influence mechanism. We further put forward the limitations of current stable isotope techniques and the future development directions.

Exogenous carbon turnover within the soil food web strengthens soil carbon sequestration through microbial necromass accumulation.

Exogenous carbon turnover within soil food web is important in determining the trade-offs between soil organic carbon (SOC) storage and carbon emission. However, it remains largely unknown how soil food web influences carbon sequestration through mediating the dual roles of microbes as decomposers and contributors, hindering our ability to develop policies for soil carbon management. Here, we conducted a 13 C-labeled straw experiment to demonstrate how soil food web regulated the residing microbes to influence the soil carbon transformation and stabilization process after 11 years of no-tillage. Our work demonstrated that soil fauna, as a "temporary storage container," indirectly influenced the SOC transformation processes and mediated the SOC sequestration through feeding on soil microbes. Soil biota communities acted as both drivers of and contributors to SOC cycling, with 32.0% of exogenous carbon being stabilizing in the form of microbial necromass as "new" carbon. Additionally, the proportion of mineral-associated organic carbon and particulate organic carbon showed that the "renewal effect" driven by the soil food web promoted the SOC to be more stable. Our study clearly illustrated that soil food web regulated the turnover of exogenous carbon inputs by and mediated soil carbon sequestration through microbial necromass accumulation.

Screening and identification of Euphorbiae pekinensis Rupr. anti-angiogenic multi-components with UPLC-QTOF-MS in zebrafish.

Euphorbia pekinensis Rupr. (EP) (Euphorbiaceae), as Traditional Chinese Medicine (TCM), exhibits therapeutic effects on tumors in clinical practice. Anti-angiogenesis may be an underlying molecular mechanism of EP's actions. However, the anti-angiogenic active ingredients of EP remain unclear. The screening and analysis of anti-angiogenic agents were essential for the sufficient utilization and development of EP. Thus, we established a UPLC-QTOF-MS method based on a transgenic zebrafish model to screen anti-angiogenesis activity components in EP. UPLC-QTOF-MS was used to characterize compounds from EP and in vivo compounds in Tg (flk1: mCherry) zebrafish larvae treated with EP. Based on the identification results, five components were selected, and their anti-angiogenesis activity were investigated via assessment of intersegmental blood vessels during the development of the transgenic zebrafish. Three of these components (3,3'-O-dimethoxy ellagic acid, quercetin, and ingenol) are active components of EP with anti-angiogenic effects. Among them, 3, 3'-O-dimethoxy ellagic acid and ingenol were first demonstrated with anti-angiogenesis effects. UPLC-PDA analysis was performed on EP water extracts to determine anti-angiogenesis active ingredients quantitatively. In the concentration range of 100-200 µg/mL, EP and the active ingredient compositions, mixed according to the content of EP, had equivalent anti-angiogenesis activities. These experimental results indicate that the UPLC-QTOF-MS method, combined with a transgenic zebrafish model, is rapid, sensitive and reliable. The combination in TCM offers the potential to achieve certain effect levels with lower concentrations of the individual compound.

Spatial patterns and ecological drivers of soil nematode ß-diversity in natural grasslands vary among vegetation types and trophic position.

Understanding biogeographic patterns of community assemblages is a core objective in ecology, but for soil communities these patterns are poorly understood. To understand the spatial patterns and underlying mechanisms of ß-diversity in soil communities, we investigated the ß-diversity of soil nematode communities along a 3,200-km transect across semi-arid and arid grasslands. Spatial turnover and nested-resultant are the two fundamental components of ß-diversity, which have been attributed to various processes of community assembly. We calculated the spatial turnover and nested-resultant components of soil nematode ß-diversity based on the ß-partitioning framework. Distance matrices for the dissimilarity of soil nematode communities were computed using the 'Sørensen' method. We fitted negative exponential models to compare the distance decay patterns in nematode community similarity with geographic distance and plant community distance in three vegetation types (desert, desert steppe and typical steppe) and along the whole transect. Variation partitioning was used to distinguish the contribution of geographic distance and environmental variables to ß-diversity and the partitioned components. Geographic distance and environmental filtering jointly drove the ß-diversity patterns of nematode community, but environmental filtering explained more of the variation in ß-diversity in the desert and typical steppe, whereas geographic distance was important in the desert steppe. Nematode community assembly was explained more by the spatial turnover component than by the nested-resultant component. For nematode feeding groups, the ß-diversity in different vegetation types increased with geographic distance and plant community distance, but the nested-resultant component of bacterial feeders in the desert ecosystem decreased with geographic distance and plant community distance. Our findings show that spatial variation in soil nematode communities is regulated by environmental processes at the vegetation type scale, while spatial processes mainly work on the regional scale, and emphasize that the spatial patterns and drivers of nematode ß-diversity differ among trophic levels. Our study provides insight into the ecological processes that maintain soil biodiversity and biogeographic patterns of soil community assemblage at large spatial scales.

Biological characterization and regulation of soil health.

How to determine the soil health status effectively is the basic issue to realize the agriculture green development. In the existing soil health assessment system, the importance of soil organi-sms in the maintenance of soil health is rarely considered. From the perspective of soil biological health, we discussed the connotation of soil health, and summarized the biological indicators of soil health, including soil microorganisms, soil enzyme activity, soil micro-food web and earthworm. Based on the above-mentioned indicators, the regulation approaches were elaborated from the aspects of crop and soil management practices. In addition, the future research on soil biological health was prospected. The main aim of this study is to enhance the understanding of scientists and decision makers on the maintenance of soil biological health, and to give full consideration of the important role of soil organisms in ecosystem services.

A global database of soil nematode abundance and functional group composition.

As the most abundant animals on earth, nematodes are a dominant component of the soil community. They play critical roles in regulating biogeochemical cycles and vegetation dynamics within and across landscapes and are an indicator of soil biological activity. Here, we present a comprehensive global dataset of soil nematode abundance and functional group composition. This dataset includes 6,825 georeferenced soil samples from all continents and biomes. For geospatial mapping purposes these samples are aggregated into 1,933 unique 1-km pixels, each of which is linked to 73 global environmental covariate data layers. Altogether, this dataset can help to gain insight into the spatial distribution patterns of soil nematode abundance and community composition, and the environmental drivers shaping these patterns.

Effect of row spacings on soil nematode communities and ecosystem multifunctionality at an aggregate scale.

Effect of crop row spacing on the belowground ecosystem, especially at an aggregate scale, remains unexplored. To explore how row spacing influenced nematode community and ecosystem function at the aggregate scale, four row spacings i.e. equidistant-row (ER, 50 cm-inter-row distance, 33 cm-intra-row between plants in each row) and non-equidistant-row including NR1 (100 cm + 50 cm row distance and 22 cm intra-row), NR2 (100 cm + 50 cm inter-row and 25 cm intra-row), and NR3 (60 cm + 40 cm inter-row and 33 cm intra-row) were compared, and four soil aggregate fractions i.e. >2 mm, 1-2 mm, 0.25-1 mm and <0.25 mm were separated. Row spacing did not impact C and N, but significantly influenced P. The regulation effect of acid phosphatase on soil available P was aggregate-scale dependent. Nematode faunal analysis indicated that NR3 within 0.25-1 mm was less disturbed or relatively undisturbed environments. Structural equation model showed row spacing pattern directly affected multifunctionality, while aggregate fractions indirectly contributed to multifunctionality mainly by regulating the richness of total nematodes and trophic groups. It was concluded that NR3 had potential to construct more stable food web, and therefore was possibly the suitable planting pattern.

Soil nematode abundance and functional group composition at a global scale.

Soil organisms are a crucial part of the terrestrial biosphere. Despite their importance for ecosystem functioning, few quantitative, spatially explicit models of the active belowground community currently exist. In particular, nematodes are the most abundant animals on Earth, filling all trophic levels in the soil food web. Here we use 6,759 georeferenced samples to generate a mechanistic understanding of the patterns of the global abundance of nematodes in the soil and the composition of their functional groups. The resulting maps show that 4.4 ± 0.64 × 1020 nematodes (with a total biomass of approximately 0.3 gigatonnes) inhabit surface soils across the world, with higher abundances in sub-Arctic regions (38% of total) than in temperate (24%) or tropical (21%) regions. Regional variations in these global trends also provide insights into local patterns of soil fertility and functioning. These high-resolution models provide the first steps towards representing soil ecological processes in global biogeochemical models and will enable the prediction of elemental cycling under current and future climate scenarios.

Effect of long-term combined application of organic and inorganic fertilizers on soil nematode communities within aggregates.

A long-term fertilization experiment was conducted to examine the effects of different fertilization practices on nematode community composition within aggregates in a wheat-maize rotation system. The study was a randomized complete block design with three replicates. The experiment involved the following four treatments: no fertilizer, inorganic N, P and K fertilizer (NPK), NPK plus manure (NPKM) and NPK plus maize straw (NPKS). Soil samples were taken at 0-20 cm depth during the wheat harvest stage. Based on our results, NPKS contributed to soil aggregation and moisture retention, with a positive effect on soil total nitrogen accumulation, particularly within small macroaggregates (0.25-1 mm) and microaggregates (<0.25 mm). The C/N ratio was correlated to the distribution of the soil nematode community. Both manure application and straw incorporation increased the nematode functional metabolic footprints within all aggregates. Additionally, the functional metabolic footprints decreased with a decline in aggregate size. The accumulation of total nitrogen within <1 mm aggregates under NPKS might play a key role in maintaining the survival of soil nematodes. In our study, both crop straw incorporation and inorganic fertilizer application effectively improved soil physicochemical properties and were also beneficial for nematode survival within small aggregate size fractions.

Distinct soil bacterial communities along a small-scale elevational gradient in alpine tundra.

The elevational diversity pattern for microorganisms has received great attention recently but is still understudied, and phylogenetic relatedness is rarely studied for microbial elevational distributions. Using a bar-coded pyrosequencing technique, we examined the biodiversity patterns for soil bacterial communities of tundra ecosystem along 2000-2500 m elevations on Changbai Mountain in China. Bacterial taxonomic richness displayed a linear decreasing trend with increasing elevation. Phylogenetic diversity and mean nearest taxon distance (MNTD) exhibited a unimodal pattern with elevation. Bacterial communities were more phylogenetically clustered than expected by chance at all elevations based on the standardized effect size of MNTD metric. The bacterial communities differed dramatically among elevations, and the community composition was significantly correlated with soil total carbon (TC), total nitrogen, C:N ratio, and dissolved organic carbon. Multiple ordinary least squares regression analysis showed that the observed biodiversity patterns strongly correlated with soil TC and C:N ratio. Taken together, this is the first time that a significant bacterial diversity pattern has been observed across a small-scale elevational gradient. Our results indicated that soil carbon and nitrogen contents were the critical environmental factors affecting bacterial elevational distribution in Changbai Mountain tundra. This suggested that ecological niche-based environmental filtering processes related to soil carbon and nitrogen contents could play a dominant role in structuring bacterial communities along the elevational gradient.

Interactive effects of elevated ozone and UV-B radiation on soil nematode diversity.

Ultraviolet-B (UV-B) radiation and elevated tropospheric ozone may cause reductions in the productivity and quality of important agricultural crops. However, research regarding their interactive effect is still scarce, especially on the belowground processes. Using the open top chambers experimental setup, we monitored the response of soil nematodes to the elevated O3 and UV-B radiation individually as well as in combination. Our results indicated that elevated O3 and UV-B radiation have impact not only on the belowground biomass of plants, but also on the community structure and functional diversity of soil nematodes. The canonical correspondence analysis suggested that soil pH, shoot biomass and microbial biomass C and N were relevant parameters that influencing soil nematode distribution. The interactive effects of elevated O3 and UV-B radiation was only observed on the abundance of bacterivores. UV-B radiation significantly increased the abundance of total nematodes and bacterivores in comparison with the control at pod-filling stage of soybean. Following elevated O3, nematode diversity index decreased and dominance index increased relative to the control at pod-filling stage of soybean. Nematode functional diversity showed response to the effects of elevated O3 and UV-B radiation at pod-bearing stage. Higher enrichment index and lower structure index in the treatment with both elevated O3 and UV-B radiation indicated a stressed soil condition and degraded soil food web. However, the ratios of nematode trophic groups suggested that the negative effects of elevated O3 on soil food web may be weakened by the UV-B radiations.

Soil nematode responses to increases in nitrogen deposition and precipitation in a temperate forest.

The environmental changes arising from nitrogen (N) deposition and precipitation influence soil ecological processes in forest ecosystems. However, the corresponding effects of environmental changes on soil biota are poorly known. Soil nematodes are the important bioindicator of soil environmental change, and their responses play a key role in the feedbacks of terrestrial ecosystems to climate change. Therefore, to explore the responsive mechanisms of soil biota to N deposition and precipitation, soil nematode communities were studied after 3 years of environmental changes by water and/or N addition in a temperate forest of Changbai Mountain, Northeast China. The results showed that water combined with N addition treatment decreased the total nematode abundance in the organic horizon (O), while the opposite trend was found in the mineral horizon (A). Significant reductions in the abundances of fungivores, plant-parasites and omnivores-predators were also found in the water combined with N addition treatment. The significant effect of water interacted with N on the total nematode abundance and trophic groups indicated that the impacts of N on soil nematode communities were mediated by water availability. The synergistic effect of precipitation and N deposition on soil nematode communities was stronger than each effect alone. Structural equation modeling suggested water and N additions had direct effects on soil nematode communities. The feedback of soil nematodes to water and nitrogen addition was highly sensitive and our results indicate that minimal variations in soil properties such as those caused by climate changes can lead to severe changes in soil nematode communities.

Nitrogen deposition weakens plant-microbe interactions in grassland ecosystems.

Soil carbon (C) and nitrogen (N) stoichiometry is a main driver of ecosystem functioning. Global N enrichment has greatly changed soil C : N ratios, but how altered resource stoichiometry influences the complexity of direct and indirect interactions among plants, soils, and microbial communities has rarely been explored. Here, we investigated the responses of the plant-soil-microbe system to multi-level N additions and the role of dissolved organic carbon (DOC) and inorganic N stoichiometry in regulating microbial biomass in semiarid grassland in northern China. We documented a significant positive correlation between DOC and inorganic N across the N addition gradient, which contradicts the negative nonlinear correlation between nitrate accrual and DOC availability commonly observed in natural ecosystems. Using hierarchical structural equation modeling, we found that soil acidification resulting from N addition, rather than changes in the plant community, was most closely related to shifts in soil microbial community composition and decline of microbial respiration. These findings indicate a down-regulating effect of high N availability on plant-microbe interactions. That is, with the limiting factor for microbial biomass shifting from resource stoichiometry to soil acidity, N enrichment weakens the bottom-up control of soil microorganisms by plant-derived C sources. These results highlight the importance of integratively studying the plant-soil-microbe system in improving our understanding of ecosystem functioning under conditions of global N enrichment.

Nitrogen addition and warming independently influence the belowground micro-food web in a temperate steppe.

Climate warming and atmospheric nitrogen (N) deposition are known to influence ecosystem structure and functioning. However, our understanding of the interactive effect of these global changes on ecosystem functioning is relatively limited, especially when it concerns the responses of soils and soil organisms. We conducted a field experiment to study the interactive effects of warming and N addition on soil food web. The experiment was established in 2006 in a temperate steppe in northern China. After three to four years (2009-2010), we found that N addition positively affected microbial biomass and negatively influenced trophic group and ecological indices of soil nematodes. However, the warming effects were less obvious, only fungal PLFA showed a decreasing trend under warming. Interestingly, the influence of N addition did not depend on warming. Structural equation modeling analysis suggested that the direct pathway between N addition and soil food web components were more important than the indirect connections through alterations in soil abiotic characters or plant growth. Nitrogen enrichment also affected the soil nematode community indirectly through changes in soil pH and PLFA. We conclude that experimental warming influenced soil food web components of the temperate steppe less than N addition, and there was little influence of warming on N addition effects under these experimental conditions.

Toxic effects of acetochlor on mortality, reproduction and growth of Caenorhabditis elegans and Pristionchus pacificus.

The effects of acetochlor on the mortality, growth and reproduction of two nematode species were assessed. The LC50 values for Caenorhabditis elegans and Pristionchus pacificus were 1,296 and 210.7 mg/L at 24 h, and 540.0 and 126.4 mg/L at 48 h exposure, respectively. In three succession generations, reproductive capacity was more sensitive in P. pacificus than in C. elegans. Moreover, the sublethal test endpoint of final length was more sensitive with P. pacificus. This study suggested that acetochlor had no long-term effects on C. elegans at lower concentrations. The higher concentrations of acetochlor (from 40 to 160 mg/L) revealed sublethal toxicity to the two tested species, with P. pacificus being more sensitive than C. elegans.

Nitrogen addition regulates soil nematode community composition through ammonium suppression.

Nitrogen (N) enrichment resulting from anthropogenic activities has greatly changed the composition and functioning of soil communities. Nematodes are one of the most abundant and diverse groups of soil organisms, and they occupy key trophic positions in the soil detritus food web. Nematodes have therefore been proposed as useful indicators for shifts in soil ecosystem functioning under N enrichment. Here, we monitored temporal dynamics of the soil nematode community using a multi-level N addition experiment in an Inner Mongolia grassland. Measurements were made three years after the start of the experiment. We used structural equation modeling (SEM) to explore the mechanisms regulating nematode responses to N enrichment. Across the N enrichment gradient, significant reductions in total nematode abundance, diversity (H' and taxonomic richness), maturity index (MI), and the abundance of root herbivores, fungivores and omnivores-predators were found in August. Root herbivores recovered in September, contributing to the temporal variation of total nematode abundance across the N gradient. Bacterivores showed a hump-shaped relationship with N addition rate, both in August and September. Ammonium concentration was negatively correlated with the abundance of total and herbivorous nematodes in August, but not in September. Ammonium suppression explained 61% of the variation in nematode richness and 43% of the variation in nematode trophic group composition. Ammonium toxicity may occur when herbivorous nematodes feed on root fluid, providing a possible explanation for the negative relationship between herbivorous nematodes and ammonium concentration in August. We found a significantly positive relationship between fungivores and fungal phospholipid fatty acids (PLFA), suggesting bottom-up control of fungivores. No such relationship was found between bacterivorous nematodes and bacterial PLFA. Our findings contribute to the understanding of effects of N enrichment in semiarid grassland on soil nematode trophic groups, and the cascading effects in the detrital soil food web.

[Effects of conservation tillage on the composition of soil exchangeable base].

Taking the soil in Zhangwu County of Liaoning Province as test object, a comparative study was made to understand the composition of soil exchangeable base under traditional tillage and 6-year conservation tillage (no-tillage plus straw mulch). Comparing with traditional tillage, conservation tillage increased the total amount of exchangeable base (SEB) and the contents of exchangeable K, Ca, and Mg in top (0-15 cm) soil, suggesting its positive effect in increasing soil nutrient holding capacity and buffering ability. This effect had a close relationship with the changes of soil organic matter and clay contents, according to correlation analysis. In addition, the K/SEB and Ca/Mg ratios were higher, while the (Ca+Mg)/SEB, Ca/K, and Mg/K ratios were lower under conservation tillage than under traditional tillage, illustrating that the effects of conservation tillage on soil exchangeable base were mainly presented in the relative enrichment of soil exchangeable Ca and K, especially K. Conservation tillage increased the stratification ratio (0-5 cm/5-15 cm and 0-5 cm/15-30 cm) of soil exchangeable K, Ca, and Mg, and SEB, suggesting the increase of the vertical variability of SEB in plough layer.

Utility of nematode Acrobeloides nanus for assessing subacute toxicity of heavy metals.

Nematodes offer perspectives for ecotoxicological research as their characters and most of toxicity assessment focused on Caenorhabditis elegans. In order to enrich the limited numbers of nematode species used for toxicity test, this study assessed the subacute effects of copper and zinc to the life history characters of nematode Acrobeloides nanus. Compared with control, the 72-h effective concentration (EC)(50), EC(20), and EC(10) for reproduction in A. nanus were 1.35, 0.49, and 0.20 mg/L, respectively, for Cu and 829.46, 330.29, and 163.90 mg/L, respectively, for Zn. The EC(10) for growth at 72 h and 96 h of the 2nd generation in A. nanus were 1.13 and 0.97 mg/L, respectively, for Cu, and 353.46 and 284.20 mg/L, respectively, for Zn. During the exposure, the effect of copper-zinc on reproduction was less than additive, and the copper-zinc effect on growth changed from a synergistic to antagonistic.

Four years of free-air CO2 enrichment enhance soil C concentrations in a Chinese wheat field.

Elevated atmospheric CO2 can influence soil C dynamics in agroecosystems. The effects of free-air CO2 enrichment (FACE) and N fertilization on soil organic C (Corg), dissolved organic C (DOC), microbial biomass C (Cmic) and soil basal respiration (SBR) were investigated in a Chinese wheat field after expose to elevated CO2 for four full years. The results indicated that elevated CO2 has stimulative effects on soil C concentrations regardless of N fertilization. Following the elevated CO2, the concentrations of Corg and SBR were increased at wheat jointing stage, and those of DOC and Cmic were enhanced obviously across the wheat jointing stage and the fallow period after wheat harvest. On the other hand, N fertilization did not significantly affect the content of soil C. Significant correlations were found among DOC, Cmic, and SBR in this study.

Effects of heavy metal pollution of highway origin on soil nematode guilds in North Shenyang, China.

Soil samples were collected with distance at 5, 20, 40, 80, 160, and 320 m from the Shen-Ha (Shenyang-Harbin) Highway, Northeast China, to investigate the effect of heavy metals of highway origin on soil nematode guilds. The contents of soil Pb, Cu, Zn, and the nematode community structure were analyzed. The results showed that the contents of total and available Pb, Cu, Zn varied significantly with the different distances from the highway. Pb was the main pollutant in the soils in the vicinity of Shen-Ha Highway. The zone from 20 to 40 m away from the highway was the most polluted area. The highest abundance of soil nematodes was found at 5 m while the lowest at 20 m away from the highway. Thirty six genera of nematodes belonging to 23 families were identified. Nematode guilds having different responses to soil heavy metals were classified into four types. Soil nematode guilds may act as a prominent indicator to heavy metal pollution of highway origin.